Down syndrome is the answer.
Take a look at the other diseases to know why.
Lupus: a chronic autoimmune disease where the person's immune system is over active and begins to affect/damage healthy tissue. It can affect tissues all over the body.
Rheumatoid Arthritis: is also an autoimmune disease, not to be mistaken with osteoarthritis (wear and tear). It occurs when the person's immune system attacks the lining of the joints, resulting in swelling and eventually joint deformity.
Type-1 Diabetes: is also an autoimmune disease. It's when the body's immune system attacks and destroys beta-cells in the pancreas. These cells produce insulin, so destroying them results in diabetes.
Down's Syndrome: Although people with down's syndrome often have autoimmune disorders, down's syndrome is a birth defect caused by a chromosome irregularity (trisomy 21). It is not an autoimmune disorder.
